Method for controlling a reduction cell

ABSTRACT

A METHOD OF OBJECTIVELY CONTROLLING AN ELECTROLYTIC CELL WHEREIN ALUMINA IS FED INTO THE CELL&#39;&#39;S BATH IN RESPONSE TO THE DETECTION OF A PREDETERMINED CHANGE OF THE CELL&#39;&#39;S RESISTANCE; BUT WHEREIN FEEDING IS PREVENTED FOR A GIVEN DWELL PERIOD AFTER EACH FEED CYCLE EVEN THOUGH SUBSE-   QUENT FEEDING CONDITIONS OTHERWISE EXIST DURING THE DWELL PERIOD.

Jan. 23, 1973 Filed lay 20, 1968 R. G. FILLER METHOD FOR CONTROLLING A REDUCTION CELL 2 Sheets-Sheet 1 ACd-l ACd-Z ANODE EFFECT/ FIG.2.

% ALUMINA CONTENT SYSTEM CONTROLLER VOLT-OHM "ETER FEED CONTROL ANODE 4 POSITION ER FlG.3.-.-

nvvewron Richard G. Piller WM M? ATTORN YS Jan. 23, 1973 R. G. PILLER METHOD FOR CONTROLLING A REDUCTION CELL 2 Sheets-Sheet 2 Filed lay 20, 1968 INVENTOR Richard 6. Pillar 1556 juso 1526 353mm 9;. 353? 35.6mm omw. 8m. cum: 0mm. mm \QO #w 000 o mw ooooooh wj a 0000 0 w 00 0000000000 00 o 32E w em 8 6 b 3 mw OONv ATTORNEYS 0 IO M Q 083 Ml 3ONV.LS I838 T133 United States Patent M 3,712,857 METHOD FOR CONTROLLING A REDUCTION CELL Richard G. Piller, Gresham, Oreg., assignor to Reynolds Metals Company, Richmond, Va. Filed May 20, 1968, Ser. No. 730,408 Int. Cl. C22d 3/12, 3/ O2; B01k 3/00 US. Cl. 20467 14 Claims ABSTRACT OF THE DISCLOSURE It has been previously suggested that aluminum reduction cells can be effectively controlled by using cell resistance in a particular manner so that it represents an objective standard for controlling the cells performance. One such method of controlling an aluminum reduction cell is described and claimed in co-pending application Ser. No. 400,059 (now abandoned) on a Reduction Cell Control System. That application was filed on Sept. 29, 1964 and is assigned to the same assignee as the instant invention.

In accordance with the present invention the cells resistance is continuously monitored, the feeding of alumina is initiated by a feed control signal responsive to cell resistance changes, and, after each feeding operation the cell is required to undergo a dwell period during which no feeding occurs even though said signal otherwise indicates that further feeding should be initiated.

Furthermore, it is the trend of the cells resistance value at any given time that determines whether the cell will be fed or not. The actual value of the cells resistance at any given time is of little significance. For example, the method of the invention might require the cell to be fed upon the occurrence of a selected number of successive resistance increases in a predetermined time period; but at another time in the cells operation, an even higher resistance might not require the cell to be fed.

In accordance with more detailed aspects of the invention, the cells AC distance is first adjusted to obtain a previously determined cell resistance (or voltage). Next, a relatively wide set of limits are established within which the cells resistance is permitted to fluctuate without any resulting changes in the AC spacing. So long as the cells resistance value remains within these limits the position of the cells anode is not changed. As the cells resistance is continually monitored, a determination is made of its minimum or track resistance, which is used to determine when the cell is to be fed. That is, the resistance during the upward trend must be above the cells most recently determined track resistance.

The foregoing and other objects, features, and advantages of this invention will be apparent from the following more particular description of preferred embodiments thereof as illustrated in the accompanying drawings.

In the drawings:

FIG. 1 is a schematic illustration of an aluminum reduction cell;

FIG. 2 is a graph illustrating the variations of atypical reduction cells resistance as its alumina content varies;

FIG. 3 is a schematic illustration of a feeding and anode control system that is suitable for use inthe practice of the method of the invention; and

3,712,857 Patented Jan. 23, 1973 ICC FIG. 4 is a graph of the variation of a typical cells resistance during the course of several typical feed cycles; and is used to illustrate the method of the invention.

The schematic illustration of FIG. 1 represents one important type of aluminum reduction cell. This type of cell is known in the industry as a prebake cell and is to be particularly distinguished from a Soderberg type of cell which is equally well known. The illustrated prebake cells anode is comprised of a plurality of carbon blocks 12. The anode of a Soderberg cell, on the other hand, is comprised of a single large mass of carbon that is baked in situ during the cells operation. Although the instant invention is applicable to either type of cell, it will be primarily described in connection with a prebake cell.

In the FIG. 1 schematic, each of the carbon blocks 12 is connected to a positive anode bus 14 by means of a suitable adjustable connection not shown.

The pot portion of the cell, which is referred to generally as 15, is comprised of a steel shell 16 which is lined with both an insulating layer 18 and a carbonaceous conductive lining 20. Iron rods 22 are embedded in the lining 20 and connected to a cathode bus 24. The lining 20 contains a pool of molten aluminum 26 and a bath 28 of alumina (A1 0 dissolved in an electrolyte comprised of molten cryolyte.

A suitable voltage forces an electrolyzing current to flow from the positive bus 14, through the carbon blocks 12, the bath 28, the molten aluminum 26, and the carbon cathode 20. The current then flows to the various current collecting iron rods 22 from which it is delivered to a subsequent cell by means of the cathode bus 24.

The molten electrolyte 28 is covered with a crust 30 which consists essentially of frozen cryolite constitutents and additional alumina. As alumina is consumed, more may be fed into the bath by either breaking in a portion of the crust 30 or using a mechanical alumina-feeder 31 in FIG. 3. As the electrolytic process progresses, molten aluminum accumulates in the pool 26 and the aluminas oxygen is combined with the anodes carbon. Conse quently, the accumulated aluminum must be periodically siphoned or tapped from the molten pool 26 and the carbon blocks must be replaced. In the prebake cells entire new carbon blocks are periodically inserted, while in Soderberg cells fresh carbon paste is added to the top of the single anode.

Customarily, as many as and more aluminum reduction cells are series connected. An ammeter such as 32 is used to measure the current through the cells; and a volt-ohm meter such as 34 is used to measure the voltage and/or resistance across each cell.

As noted above, the method of the instant invention contemplates periodic adjustment of the cells anode at various times and for various reasons during the course of the cells operation and feeding cycles. In addition, the instant invention requires that the cell be operated so as to provide feeding cycles of one or another predetermined type wherein each such feeding cycle is followed by a mandatory dwell period during which the cell is not permitted to be fed.

As illustrated in FIG. 2, for a given AC distance, a minimum cell resistance corresponds to a particular alumina content, such as, for example, 5% in FIG. 2. As the cells alumina content increases, its resistance also increases at a negative slope, i.e., sloping upwardly and to the left in FIG. 2. When the cells alumina content becomes too large, sludging occurs and the cell is referred to as becoming sick. The cells resistance also increases as its alumina content decreases. Eventually, a point is reached where the cell enters a condition which is commonly referred to as an anode effect. This resistance increase,

however, has a positive slope. In this respect, it should be noted that after a cell has received a heavy feeding such as by a massive crust breaking, its resistance must first decrease toward the minimum point 38 before it again increases as the cells alumina content decreases toward the 2% content in FIG. 2. Consequently, by virtue of this methods requirement for a mandatory dwell period, a cell operator can determine whether a given resistance rise is due to a low alumina content merely by determining whether the resistance has first gone through a relative minimum value. This simple concept has been masked by previous control methods which employed substantially continuous feeding techniques without providing an opportunity for the cell to adequately accommodate all of the alumina that it had previously beed fed.

The method of the invention can be practiced by means of apparatus such as that which is schematically illustrated in FIG. 3. Therein, the carbon anode blocks are moved upwardly or downwardly by means of an anode positioning mechanism 40 which is operative in response to signals from a system controller 42, whose sophistication can range from that of a mere switch panel to that of a computer. In fact, in some cases, it may even be satisfactory to position the anode manually. A conventional alumina feeder 31 is controlled by a feed controller 44 which, in turn, is responsive to the system controller 42. Although many types of mechanical alumina feeders are suitable for use in the practice of the invention, it has been found convenient to use a type which punches through the crust 30s central portion at selected time intervals so as to introduce similarly selected amounts of alumina into the cells bath 28.

Periodically, the cells amperage and voltage are sampled and the cells resistance is determined. In one situation where the invention has been practiced, for eXample, the cells resistance was determined about every three minutes. In fact, four rapid resistance measurements were made about every thirty seconds; their values averaged; and the three minute resistance determinations were the result of the average resistance values after they had been subjected to an exponential smoothing operation.

It has been noted that a primary aspect of the invention relates to an objective method of cell control wherein the initiation of a selected feed cycle is determined by the trend of the cells resistance; and where the various feed cycles are separated by a predetermined minimum dwell period. Before discussing that aspect of the invention in more detail, however, the cells anode control aspects 'will first be described.

Reduction cells have been conventionally controlled by both periodically breaking in the crust to introduce alumina into the bath, and by periodically adjusting the AC spacing so as to obtain a set voltage or set resistance. Both of these control steps have the effect of changing the cells resistance. AC spacing changes vary the electrical resistance path through the bath between the anode and cathode; and the crust breaks vary the electrical resistance baths dissolved alumina content.

For years, cell operators have been empirically determining a given cells set resistance, but recently such set resistance values have been calculated by more objective means. Customarily, a cells set resistance is based upon a host of factors such as the cells age; the type of alloy being produced; the cells temperature history; and so on. These factors are well known in the art and will not be further discussed at this time. Hence, suifice it to say that a cell is initially placed in operation by adjustin its anode so as to obtain a particularly determined set resistance. This value is illustrated by the set resistance line 50 (R in FIG. 4.

In the past, a cells operator would monitor the cells set voltage and make frequent anode adjustments in an attempt to maintain a given AC distance throughout various periods of the cells operation. A given AC distance, however, is not susceptible to being accurately obtained by means of mere anode adjustment alone. As will be more fully appreciated shortly, the method of the invention is based upon a type of resistance control that permits far more accurate control over AC spacing.

As illustrated in FIG. 4, the set resistance line 50 (at 41.50 micro-ohms) is bounded by an upper limit boundary line 52 at 43.50 micro-ohms and a lower limit boundary line 54 at 41.00 micro-ohm. These are merely illustrative values, however, because the invention has been practiced with boundary limits that were as much as 10% above the cells set resistance (R and 7% below R On the other hand, as will be discussed later, it has been found that better control is sometimes obtained by selectively reducing these limits. During the above noted testing operations, however, the preferred normal limits have been found to be about 3% of R for the lower limit and about 4% of R for the upper limit.

Insofar as AC distance changes are concerned, a major distinction between this invention and that of past control methods, is that while cell operators formerly chained anodes to maintain a given set voltage, this in'ventions method permits the cells resistance to fluctuate between these upper and lower boundary limits. But, when the cells instantaneous resistance exceeds those boundary limits (such as at point 64 in FIG. 4), the cells AC spacing is changed to make its actual resistance more nearly agree with the set resistance. This will become more clear as the discussion of FIG. 4 progresses, but first the feeding aspects of the invention will be discussed in more detail.

In order to obtain true objective cell control, it is necessary both that the control criteria be susceptible of accurate determination and that the cells parameters be capable of being varied in a manner so as to meet these criteria. From the above discussion, it should be clear that mere control of the cells anode position is not satisfactory. Consequently, the method of this invention includes a close monitoring of the cells resistance; and feeding the cell at a time when its resistance is determined to be in a positively sloping upward trend; but requiring each feeding step to be followed by a dwell period during which no further feeding is permitted. This type of operation is illustrated in FIG. 4, wherein the line 59 represents the variations of the cells resistance at various times during a given cells continuous operation in accordance with the method of the invention. The circles 60, on the other hand, represent the cells track resistance (R or R for corresponding periods of time (T). In this respect, the cells track resistance represents an effort to detect or track the cells minimum resistance point such as that represented by the point 38 in FIG. 2.

As will be illustrated shortly, a new minimum resistance (track resistance) is taken as being established any time that two successive resistance values on line 59 are less than the existing track resistance, irrespective of the relative magnitudes of those two succeeding resistance values. In other words, a new R (R is established if:

in which even R =R or; if preferred, the lesser of R or R In addition, in some cases it is desirable to change the cells assigned R value even though it is not yet followed by two successively lower resistance values. This would occur, for example, whenever the cells resistance profile is drastically altered, such as when the cell operator makes a major change in the anode position, or after the completion of a feeding cycle.

At time zero (T the illustrated cells set resistance (line 50) was 41.50 micro-ohms; its track resistance was about 42.00 micro-ohms; and its actual resistance was about 42.50 micro-ohms. At that time, the cell was being fed; and the feeding cycle did not terminate until time 3(T This was followed by a dwell period from T to and T and according to the method of the invention, the cell could not be fed during a dwell period.

As the cell entered the dwell period its anode was ad justed to bring its actual resistance into agreement with R This is indicated by the downwardly sloping arrow 61 extending from resistance point 62 at T to resistance point 63 at T and because of this change in the cells resistance profile a new track resistance of about 41.63 micro-ohms was established at T At T the cells actual resistance at point 64 fell below the R lower limit line 54 so that a signal (or group of signals) was delivered by the system controller 42 to the anode positioner 40 so as to raise both the anode and the cells resistance to an actual resistance value illustrated by point 65 which is somewhat higher than the previously established set resistance. Because of this change in the anode position, the cells track resistance was again redetermined. In this respect, it should be noted that track resistance provides a frame of reference for determining when a feed cycle should be initiated. Hence, during feed cycles (or special events such as tapping and anode effects) the track resistance becomes somewhat meaningless and can be terminated. But, after each feed cycle (or special event) the AC spacing is changed as required to bring the actual resistance (R into within about 1% or so of agreement with the set resistance (R Hence, in accordance with the above principles, the cells track resistance is redetermined at such times to account for this change in the cells resistance profile. In fact, inasmuch as the track resistance is not monitored during feeding it is a simple and preferable matter to redetermine track resistance after each feed cycle whether or not the AC spacing is changed.

In the above respects, it has been found convenient to merely arbitrarily redetermine the cells track resistance as being equal to its present actual resistance value; and then continually update that R value if the cells resistance subsequently decreases. This is illustrated at point 65 which occurred at T One of the inventions criteria for determining the time at which a cell is to be fed is that a given actual resistance value R be both higher than the cells most recently established track resistance and the preceding actual resistance value (R as well as being lower than the cells next actual resistance value (R In other words a feed signal is generated when:

In accordance with both the requirements of those criteria and the method of the co-pending application, a feed signal would be generated at T in FIG. 4. But because of the instant inventions requirement for a mandatory dwell period, such a feeding cycle was not initiated during the operation of the illustrated cell. That is, a dwell override signal from the controller, in effect, cancelled the feed signal generated at T Inasmuch as the required dwell period did not terminate until T therefore, a feed cycle could not be initiated until that time. In this respect it should be noted that the actual resistance point 68 also meets the above noted criterion for the initiation of a feed cycle. That is, point 68 is greater than both the cells track resistance (R and its preceding actual resistance (R as well as being less than its next succeeding actual resistance value at point 70 Of course, point 68s satisfaction of that criterion could not have been determined until after point 70 was established. Hence, a feed cycle was not initiated until T -a time after which the controllers dwell override signal had terminated.

The duration and composition of a given feed cycle depends upon many factors, not the least of which is an and analysis of the historical peculiarities of a particular cell. For example, older cells have a tendency to take alumina into solution somewhat faster than others. In this respect, the feed cycle extending between T and T might have been comprised of seven punches by the feeder mechanism 31, at 100-second time intervals; followed by eleven punches at l-second intervals; in which event the entire feed cycle would have extended over a 45 minute period.

Another factor to be considered in connection with the format of a particular feed cycle is the rate at which the cells actual resistance is increasing. For example, if the cells actual resistance value at T had been at point 70', it might have been more desirable to feed the cell both more rapidly and for a longer period. In that case, the feeder 31 might have been controlled so as to provide eleven punches at 70-second intervals followed by perhaps thirteen punches at 180-second intervals. In this manner, the cell would have received 6 additional charges in the course of a 52 minute period. Having had the benefit of the above described illustrations of various feeding-cycle formats, it is believed that additional suitable feedingcycle formats can be similarly established by men skilled in the art. Consequently, other types of feeding formats will not be further discussed.

By T the effects of the just concluded feeding cycle were evident by virtue of the cells actual resistance having dropped to about 42.00 micro-ohms at point 72just above the cells previously reestablished minimum resistance R (R By T however, the cells actual resistance was below its previous R, value; and by T was still lower. A new R, was established, therefore, at R by virtue of the former R s value being followed by two lower resistance values (R and R That is, the requirements of Equations la and 1b had been met. This new minimum resistance value at T is illustrated as being at about 41.63 micro-ohms in FIG. 4.

R in addition to being higher than R was also consecutively progressively higher than R Hence, Equations 2a and 2b were satisfied and met the requirements for the initiation of a feed cycle at T Again, however, the feed signal occurred during a required dwell period so the initiation of a new feed cycle was not permitted. At T the actual resistance value was again below the most recently established R, value; and at T the actual resistance remained below the established level of R ('R Consequently, even though R was above R a new R, was established because R was followed by two consecutively lower values (even though not progressively lower). Moreover, this most recently established R, was again superseded when the same criteria were met by the cells actual resistance at point 78 occurring at T The required dwell period extending between T and T ended at a time when two of the cells consecutive actual resistance values were higher than the R level established at point 78. Inasmuch as point 80s actual resistance was the same as the immediately preceding value, however, Equation 2bs feed criterion was not met. At T32, however, a feed cycle was initiated because at that time (T the preceding point 81 was established as being higher than both R and point 80, but lower than point 8 2.

It should be particularly noted that the cells resistance at point 82 (41.87 micro-ohms) was considerably less than the cells resistance at point 70 (43.00 micro-ohms). This is significant because feed cycles were begun at both of those resistance levels even though there were no ob served intervening changes in the cells AC distance.

The feeding cycle which began at T terminated at T at which time the cell entered another mandatory dwell period. At T the track resistance was arbitrarily reestablished to coincide with the actual resistance at point 83, but by T another new track resistance was established at point 84; and this was followed by still lower track resistance values at points 85 and 86 (T and T respectively) The mandatory dwell period ended at T and a new feed cycle was initiated at T even though the requirements of Equations 2 were not satisfied. This was because a second set of feed criteria were met. That is In other words, the increase in the cells actual resistance was so great in such a short period of time that the cell was probably heading for an anode effect if it were not fed immediately. Although sometimes helpful to prevent anode effects, these later criteria have been eliminated in some cases without particularly detrimental effects upon the cells operation.

As noted above, it is sometimes considered necessary to break in a considerable portion of the cells crust. For example, in some instances it is customary to break in half or all of the cells crust during each shift. This operation decreases the possibility of the pot becoming contaminated such as by unburned carbon that has accumulated in the bath. When the cells crust is thusly broken in, however, this carbon is permitted to burn ofl. At such a time therefore, there is no need to be concerned about either feeding the cell or tracking. Consequently, after each crust break, the cell is also placed in a mandatory dwell period. In addition, because the tracking operation has been terminated and thereby no longer controls feed, more reliance is placed on anode control. Hence, after a crust break, it has been found convenient to decrease the upper limit 52 of the set resistance during this succeeding dwell period.

In the event that a cell does undergo an anode effect, its actual resistance goes very high because of the extremely rapid increase in its over-voltage. Clearly, this is not due to an improper AC distance. Hence, during an anode effect, no attempt is made to decrease the cells resistance by lowering its anode. After an anode effect, the upper limit line 52 of the set resistance is eliminated for some suitable period of time, such as 30 minutes after the anode effect has terminated.

It should be appreciated, therefore, that although the method of the invention contemplates the establishment of normal R boundary limits for control during normal cell operation, the R boundary limits can be suitably altered in the event of crust breaks, anode effects, metal taps or the like.

While the invention has been particularly shown and described with reference to preferred embodiments thereof it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. For example, although the invention has been described in terms of measuring and plotting certain variables, it will be apparent to those skilled in the art that analog or digital computers can be used to perform these functions with much more facility and rapidity. In fact, a computer has been very effectively used to carry out the principles of the invention with a minimum amount of time and effort so as to obtain a practical maximum efficiency level from the alumina reduction cells that were thusly operated.

Similarly, it will also be appreciated that it is usually desirable to place certain tolerance requirements on the instrument readings of the various cell parameters. For example, depending upon the type of instruments employed, it might be desirable to require a given actual resistance value to exceed certain tolerance limits before t would be considered to be less than R, for purposes of establishing a new track resistance.

What is claimed is:

1. In the operation of an electrolytic cell for the reduction of alumina dissolved in a molten salt bath, by passing electric current through said bath between a cathode and a relatively adjustable anode, including the steps 0f periodically adjusting the cells electrode spacing and regulating the feeding of alumina into the bath in response to changes in cell resistance during the interval between successive adjustments of the electrode spacing, whereby the cell resistance is affected by changes in AC distance of the cell as well as by variations in bath alumina concentration arising from reduction of alumina that is fed to the cell from time to time, the method of con trolling subsequent feeding of alumina into said bath after a prior feeding operation comprising the steps of:

providing a dwell period of predetermined minimum duration following said prior feeding and continuing the operation of said cell without feeding during said dwell period;

providing a track resistance reference signal corresponding to the value of cell resistance at a given time and updating said signal in response to decreasing cell resistance;

producing a feed control signal effective after termination of said dwell period to indicate a condition of increasing cell resistance in accordance with a predetermined criterion relative to a lower value thereof represented by the then existing track resistance reference signal; and

initiating said subsequent feeding in response to said feed control signal.

2. The method of claim 1 including the steps of:

adjusting tht AC distance to obtain cell resistance corresponding to a predetermined set resistance value for said cell;

establishing a range of cell resistance values above and below said set resistance value; and

permitting said cell resistance to vary within said range of cell resistance values without readjusting said AC distance, but readjusting said AC distance when the value of said cell resistance exceeds said range of cell resistance values.

3. The method of claim 2 wherein:

said cell resistance range extends from a value not exceeding about 10% higher than said set resistance value to a value not exceeding about 7% lower than said set resistance value.

4. The method of claim 1 wherein said cell resistance is determined at regular intervals and is determined to have decreased during a given time interval so that said track resistance reference signal is thereby updated, when:

both the cell resistance during said given interval and the cell resistance during the next preceding interval are less than the value of cell resistance corresponding to the then existing track resistance reference signal.

5. The method of claim 4 including the step of:

determining that said criterion has been met when cell resistance at a given time is:

(i) greater than both the cell resistance corresponding to the then existing track resistance reference signal and the cell resistance determined during the next preceding time interval, but

(ii) less than the cell resistance that is determined during the next time interval.

6. The method of claim 1 including the step of:

re-establishing said track resistance reference signal to correspond to a value substantially equal to cell resistance after each feeding.

7. The method of claim 1 wherein cell resistance is determined at regular time intervals and wherein subsequent feeding is initiated after a determination that:

the rate of change of cell resistance over a particular time interval is greater than the rate of change over the next preceding time interval.

8. The method of claim 1 including the steps of:

establishing a predetermined set resistance value for said cell; and

adjusting the AC distance after feeding as required to obtain a cell resistance value close to said set resistance value.

9. The method of claim 8 wherein the AC distance is adjusted to obtain a cell resistance within about 1% of said set resistance value.

10. The method of claim 1 wherein said feeding includes a series of separate feeding steps separated by a selected feed step time interval, each of said steps providing a selected amount of alumina.

11. The method of claim 10 wherein the number of feeding steps and the time intervals therebetween are determined in accordance with the rate of change of cell resistance prior to the initiation of said feed cycle.

12. The method of claim 1 wherein the cell resistance is periodically determined by the steps comprising:

rapidly sampling the cells current and voltage a first number of times and determining a first group of cell resistance values;

averaging said first group of cell resistance values to obtain a first average cell resistance value;

rapidly sampling the cells current and voltage at least a second and third number of times to determine at least second and third groups of cell resistance values;

averaging at least said second and third groups of cell resistance values to obtain at least second and third average cell resistance values; and

exponentially smothing the thusly determined average cell resistance values.

13. In the operation of an electrolytic cell for the reduction of alumina dissolved in a molten salt bath, by passing electric current through said bath between a cathode and a relatively adjustable anode whereby the cell resistance is alfected by changes in AC distance of the cell as well as by variations in bath alumina concentration, the method which comprises:

periodically adjusting the cells electrode spacing to 10 maintain the cell resistance within a control range having prescribed upper and lower limits; and regulating the feeding of alumina into the bath in response to changes in cell resistance during the interval between successive adjustments of the electrode spacing wherein said regulating includes:

(a) feeding alumina into the bath;

(b) providing a track resistance reference signal corresponding to the value of cell resistance at a given time and updating said signal in response to decreasing cell resistance as operation of the cell proceeds following said feeding;

(c) providing a feed control signal for indicating the occurrence of two successive increases in cell resistance relative to a lower value thereof represented by the then existing track resistance reference signal; and- (d) initiating subsequent feeding in response to said feed control signal.

14. In the operation of an electrolytic cell for the reduction of alumina dissolved in a molten salt bath, by passing electric current through said bath between a cathode and a relatively close to said set resistance value; and

re-establishing said track resistance reference signal to FOREIGN PATENTS 9/196T U.S.S.R.

JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner U.S. C1. X.R. 

